The present invention relates to a method for the surface treatment of load-bearing surfaces in steel casings, oil or gas field tubes, or the like.
Casings and tubing of the type to which the invention pertains are used in the oil and gas industry and are to be provided with a thread or threads so that these casings or tubings can be threadedly interconnected with each other or with sleeves. The load-bearing surface portions of the threading or the entire surface of the threaded portion of such a casing, tube or pipe must be particularly treated in order to counteract excessive wear when the joint is made up. The same is true with regard to specially provide sealing surfaces and shoulders which will enter into metal-to-metal engagement with suitably contoured counterparts in the respective other tube, casing, sleeve, or the like, in order to obtain a sealed connection. These sealing surfaces are also required to be particularly treated because, in any of these cases, it is necessary to provide surface properties which will result in a reduction of the sliding resistance of the respective parts so that, upon threading them together, the sliding resistance is greatly reduced.
It is quite apparent that threaded and sealing connections of the type outlined above experience a very high percent of surface unit area loads. These loads are required simply because, in order to obtain the requisite firm and sealed connection, the engaging surfaces must be very forcefully pressed together. On the other hand, many of these threaded connections have to be released upon unthreading the tubing and sleeves from each other; moreover, these threaded connections, sealing surfaces, etc., have to be reused in the same capacity, possibly over and over again. If the respective high-load-bearing surfaces are not adequately treated, in other words, if they pertain to the type of steel which is normally being used for such casings and tubes, these surfaces will score as they slide against and over each other. Such a steel may be of the low-alloyed variety or of the high-alloyed variety with additions of chromium, nickel, molybdenum, and other additives; here, in particular, one has to expect that in the normalized or dressed condition, these alloy components are subject to scoring, welding, and other mutual damage.
The simplest mode of treating the surfaces of the tubes and casings of the type mentioned above is to deposit high-pressure grease upon the surfaces. In accordance with another proposal, the surface expected to experience a high load is treated chemically, or another metal is deposited or plated on. Also, the so-called phosphating is a method which falls within this class. All of these known methods have been found to be unreliable for one reason or another.